Is it obligatory for every Muslim to believe?

Faith has several meanings. If used absolutely, it includes Islam, and it is obligatory, as Islam or faith cannot stand without it. If mentioned alongside Islam, what is meant is the inner creed, and it is obligatory, as faith or Islam cannot be sound without it. It may also refer to the perfection of faith, as in the verse: "Indeed, the believers are those who, when Allah is mentioned, their hearts tremble, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely." [Al-Anfal: 2]. Of this, some is obligatory (obligatory perfection of faith), and some is recommended (recommended perfection of faith).